Background/Objectives:
Spinal muscular atrophy (SMA) is a genetic disorder resulting in weakness and impaired function. Individuals with SMA are at risk for low bone mass (LBM). In children, non-ambulant SMA have decreased bone mineral density (BMD) compared to ambulant SMA. SMA mouse models suggest function and intrinsic mechanisms impact bone development and remodeling. Less is known about the relationship between BMD and function and activity in SMA.
Methods:
Thirty-five children and adults participated (19 ambulatory SMA,16 controls). Dual-energy absorptiometry determined BMD. LBM was identified using a T-score ≤ -1 in ≥ age 50 or a Z-score ≤ -2 in < age 50. The six-minute walk test (6MWT) and Timed Up and Go (TUG) evaluated function and the International Physical Activity Questionnaire evaluated physical activity (PA).
Mann-Whitney tests were used for group comparisons. Univariable logistic regression and Spearman rank coefficients determined factors associated with BMD. Receiver operating characteristic curves assessed the ability of the measures of function and PA to discriminate between individuals with and without LBM.
Results:
SMA participants had lower BMD compared to controls (p<0.001). Ten SMA participants and 1 control had LBM. Total Z-score was associated with 6MWT (r=0.65; p<0.001) and TUG (r=-0.61; p<0.001). There was an increased odds of having LBM in individuals with SMA (OR=16.67; 95%CI: 1.82–152.77; p=0.004). A 6MWT (AUC: 0.80; p=0.006; CI: 0.65-0.94) of 456.6m or less or a TUG (AUC: 0.85; p=0.002; CI: 0.71-0.98) of 7.27s or more identified individuals with LBM with sensitivity of 91% and 90% and specificity of 71% and 82% respectively.
Conclusions:
Function is associated with BMD and can be used to identify individuals with LBM. Clinical practice should emphasize strategies that improve function and mitigate the negative sequelae associated with LBM. Future research should examine the influence of SMN insufficiency on bone and identify interventions that may improve BMD in SMA.